Cultural Symbol - This pony specializes in the ponies of the symbol's source. This does, however, mean their skills are rather limited when it comes to other cultures.
Entertainment - A more general symbol of some form of entertainment. These ponies do make reasonable diplomats, but tend to brute-force things a little too often. (Pinkie Pie, Octavia, Vinyl Scratch)
Animal - A creature of some sort. Ponies who can understand and be understood by all the non-pony creatures find themselves bridging gaps all around them. (Fluttershy)
Sport - This pony makes a great athlete, and is reasonably fit. Unfortunately, their mindset is set up for competition, not life and death struggle, so their value in war is minimal.
Weather Symbol - Clouds, lightning, and all sorts of storms. It's rough in the skies, and ponies marked by the weather tend to be tougher than most. (Rainbow Dash, Lightning Dust)
Law Symbol -
Weapons/Armor - This pony excels at protecting their friends. Whatever it takes, these ponies are willing and able to do it. (Shining Armor)
Glittery Object - While this pony has some grasp of the value of money, they appear more suited at spending it freely than making it. (Diamond Tiara, Silver Spoon)
Food - Fruits, vegetables, flowers, or something prepared in a kitchen. A pony who knows how to get food on the table can always find work. (Apple clan, Carrot Top)
Valuable Material - Frequently gemstones, but metals also. Ponies with these on their flanks are well-skilled to run profitable businesses. (Rarity)
Money - The symbols of economic strength grace the flanks of ponies who can find profits in even the most humble of settings. (Filthy Rich)
Navigation Object - Maps, compasses, and similar devices. A pony can learn a lot just from the shape of the land, but not everything. (Daring Do)
Historic Symbol - A symbol that was used in older times, but has gone into disuse since. Ponies with this tend to be quite familiar with the context and use of their symbol, but their knowledge in every other area is lackluster.
Writing - Books, scrolls, and even writing implements. This pony can be quite organized and knowledgable, but don't have the knack for research.
Scientific Instrument - From telescopes to hourglasses, these ponies have keen minds and a naturaly methodical approach to study. (Dr. Whooves)
Celestial Object - Sun, Moon, Stars. Ponies with this type of mark invariably have a much deeper understanding of the world and magic. (Celestia, Luna, Twilight Sparkle)